The White House threatened further sanctions against Russia Monday after pro-Russian activists in eastern Ukraine seized government buildings and called for a referendum on seceding from the embattled country.

The activists barricaded themselves inside the provincial administrative building in Donetsk -- a city bordering Russia -- over the weekend, and announced Monday the formation of the independent Donetsk People's Republic. The demonstrators called for a referendum to be held no later than May 11, Interfax reported.

Sky News reported that dozens of people armed with sticks and rocks broke through police lines during a pro-Russian rally and surged into the government office in Donetsk, roughly 50 miles west of the Russian border.

Some of the protesters were chanting "Donetsk is a Russian city" as they climbed to the roof, where they waved the Russian flag. They then threw firecrackers at around 200 police officers surrounding the building. Local media reported that groups occupying the provincial government building have barricaded the entrance with car tires and barbed wire.

The aggressive action is causing international fears of a repeat of Russia’s annexation of the Ukrainian peninsula of Crimea last month.

The Ukrainian government promised to handle the situation, but Russia warned Ukraine of more "difficulties and crises" if its leaders failed to heed Moscow's demands. Tens of thousands of Russian troops are stationed along the border with Ukraine.

Speaking at an emergency Cabinet meeting, Prime Minister Arseniy Yatsenyuk said Russia's plan was "to destabilize the situation; the plan is for foreign troops to cross the border and seize the country's territory, which we will not allow." Yatsenyuk also claimed that people engaged in the unrest have distinct Russian accents.

Sources in Ukraine tell Fox News the Russian Foreign Ministry released a statement Monday stipulating its previous demands to change its constitution to turn the country into a federation, and ensure a special status for the Russian language.

White House spokesman Jay Carney told reporters Monday that the U.S. is watching the situation closely and ready to respond.

“In order to resolve and deescalate the situation, Russia needs to move those troops back from the border region, and to begin negotiations directly with the Ukrainian government. As I noted before, we are prepared to impose further sanctions on sectors of the Russian economy should the situation escalate,” Carney said.

There was strong evidence that some pro-Russian protesters in eastern Ukraine were paid, Carney said. “They (the demonstrators) were not local residents. I think that this suggests that outside forces, not local forces, were participating in the effort to create these provocations,” Carney added.

In Kiev, President Oleksandr Turchinov described the developments in eastern Ukraine as an operation undertaken by Russia to sow instability.

"Anti-terrorism measures will be adopted against those who took up weapons," Turchinov said, adding that parliament would convene Tuesday to consider tougher penalties for separatist actions and a ban on separatist parties.

Ukrainian Prime Minister Arseniy Yatsenyuk also accused Russia of staging the unrest to create a pretext for sending troops across the border.

"The plan is to destabilize the situation, the plan is for foreign troops to cross the border and seize the country's territory, which we will not allow," he said, adding that those taking part in the unrest had distinct Russian accents.

Meanwhile, outside the Donetsk building, a barricade of car tires and razor wire was built up to prevent police from retaking it. Other armed activists fired warning shots in the air and tried to seize the regional state television building in Donetsk, but retreated Monday after police and guards in the building also fired warning shots in the air, Interfax cited police as saying.

Sources in Ukraine tell Fox News clashes have been reported in the Russian-speaking city of Kharkiv. Gangs of separatists have attacked and violently dispersed a pro-Ukrainian demonstration, and top government officials headed there to try to quell the unrest.

There are also reports from Luhansk that a regional administration building has been sealed off, and some roads to the city have been blocked by armed separatists. As of late Monday, the buildings in all three cities remained occupied by pro-Russian activists whom authorities said were armed.

The Russian Foreign Ministry rejected the Ukrainians' allegations, but reaffirmed its long-held demand that Ukraine should change its constitution to turn the country into a federation with broader powers for provinces. It added that such moves should also underline Ukraine's non-aligned stance and ensure a special status for the Russian language.

"If the political forces that call themselves the Ukrainian government continue to take irresponsible attitude to the fate of the country and its people, Ukraine will inevitably face new difficulties and crises," the ministry said in a statement.

Eastern Ukraine, which has a large population of ethnic Russians, was the support base for ousted President Viktor Yanukovych, who fled to Russia in February after months of protests. Both economic and cultural ties to Russia are strong here and many are wary of the new government, which favors closer ties to the European Union.

NATO says up to 40,000 Russian troops have mobilized and present a distinct threat to Ukraine. Russia says it has the right to move its troops wherever it wants on its own territory.

The Associated Press contributed to this report.

Click for more from Sky News